1. Field of the Invention
The present invention relates to radio location systems and, more particularly, to radio location measurement units used in radio location systems for locating the positions of cellular radio telephones (mobile stations).
2. Description of the Related Art
There are many systems known in the art by which the position of such a mobile station can be determined. Of particular relevance here are those systems for locating mobile stations in mobile telephone communications networks. One such method, known by its standardised acronym as E-OTD (Enhanced-Observed Time Difference) uses the relative timing offsets of signals received from the network transmitters by a mobile station, together with the relative timing offsets of the same signals received by a fixed receiver whose position is known. The second set of measurements by the fixed receiver is required since the transmissions may not be synchronised with respect to each other so that their relative transmission time offsets (i.e. the offsets in the times at which identical parts of the signals are transmitted from different transmitters) are constantly varying and otherwise unknown.
Two principal, and different, methods of using the timing offsets in the position computation have been described in the art. In one, e.g. EP-A-0767594, WO-A-9730360 and AU-B-716647, the details of which are hereby incorporated by reference, the signals measured by the fixed receiver are used, in effect, to ‘synchronise’ the transmissions from the different transmitters. The instantaneous transmission time offsets of each transmitter relative to its neighbours are calculated from the values measured at the fixed receiver using the known positions of the fixed receiver and the transmitters. The timing offsets measured by the mobile station can then be used in a calculation based on well-known standard techniques in which the points of intersection of two or more hyperbolic position lines predicts the position of the mobile station.
The other method (see our EP-B-0303371, U.S. Pat. No. 6,094,168 and EP-A-1025453 the details of which are hereby incorporated by reference and which refer to a system known as CURSOR®) makes use of the measurements made by both the fixed receiver and the mobile station to calculate the relative time difference between the signals received from each transmitter by both receivers. This results in a calculation based on the intersection of circles centred on the transmitters.
In our WO-A-0073813, the details of which are hereby incorporated by reference, we have shown how the E-OTD technique can be further refined for large networks by combining the measurements from two or more of the fixed receivers (the so-called Location Measurement Units: LMUs), each of which can only receive signals from a subset of the transmitters in the network, to produce a list of the measurements that would have been provided by a single unit, the Virtual LMU (VLMU) had it been able to receive transmissions from the entire network. This technique may be a required element of any practical implementation of E-OTD.
All E-OTD systems require the measurement of the times of arrival of radio signals from at least three transmitters at both the mobile station (MS) and at least one LMU whose position is known or can be calculated.
It is convenient to install the necessary LMUs at (i.e. co-located with) the existing base transceiver stations (BTSs) which are used to transmit and receive the communications signals to and from the mobile stations. By doing so the network operators avoid the need to obtain additional costly sites for their LMUs, and also have access to the site services and communications channels used by the BTS itself. However, field trials have shown, surprisingly, that signals from some distant BTSs are often more reliably received by an LMU than those from its co-located BTS. Investigations have shown that, in particular, it is difficult to place the LMU antenna so that all of the locally transmitted channels are received properly at the same time as receiving the signals from the distant BTSs. Further examination indicates that this problem is not as a result of signal strength levels, which are always strong in the vicinity of the BTS (although not usually so strong as to block the receipt of other signals). Instead, the problem is caused by the interference effects from “local scatterers”, such as the ground and nearby buildings, which often cause the received signals to be delayed by substantial, variable, and unknown amounts. This leads to the significant problems that (a) the VLMU must rely on measurements of the ‘local’ BTS signals made by ‘distant’ BTSs with the consequent uncertainties in the propagation paths, and (b) in the case that GPS or some other ‘absolute’ timing reference is being used, the ‘local’ signals cannot be accurately measured. The result is that, if coverage of the local BTS cells cannot be guaranteed by the co-located LMU, it becomes very difficult for the location system to guarantee coverage of the entire BTS network.
There is a need therefore to overcome this problem if the advantages of co-locating LMUs and BTSs are to be retained.